Nasa's planet-hunting Kepler mission may be on its last legs

An artist's conception of Kepler-62e, a possibly habitable, water-covered planet discovered by Kepler in April 2013

NASA/Ames/JPL-Caltech

Problems with one of Kepler's reaction wheels, which are used to
aim the telescope, have dogged the mission since January, and it
could now be in danger of shutting down.

The $600 million (£385 million) telescope, launched in March 2009, is used to detect planets orbiting
far-off stars. It monitors over 150,000 stars and during its
four-year lifetime has detected 2,700 possible exoplanets.

Kepler uses an array of 42 charge coupled devices to detect
minute variations in the light from distant stars that occur when
orbiting planets transit in front of the stars. By measuring how
long it takes the planet to pass in front of the star, and the dip
in brightness the planet causes, the size of the planet's orbit and
the planet's mass can be calculated.

Kepler originally began the mission with four reaction wheels:
three for manouvres and one spare. In 2012, reaction wheel number
twoexperienced six months of elevated friction levels and then
failed in July of that year.

"The part that worries us is that the elevated friction that
we're seeing in wheel number four now is very reminiscent of what
we saw a year ago in wheel number two, which eventually failed,"
Kepler deputy project manager Charlie Sobeck told Space.com.

"The engineering team has now turned its attention to the
development of contingency actions should the wheel fail sooner,
rather than later," wrote Roger Hunter, Kepler mission manager, in
an update on 29 April.

Potential contingency actions could include using Kepler's
thrusters in conjunction with the two remaining wheels, but this
would have the negative side effect of running down Kepler's fuel
reserves more quickly than planned. The engineers are also
exploring ways to bring wheels two and four back online.

While the loss of another wheel would halt the current mission,
which relies on Kepler remaining focussed on one area of sky, the
telescope could still be used in other ways, like scanning across
the sky.

Kepler was originally planned to be operational for three and a
half years, but has already exceeded its initial mission and had been expected to stay in
operation until 2016.